Lipid membranes, present in all three domains of life, serve the crucial function of separating chemicals of cellular life from an aqueous environment. Thus, the lipid world has been proposed as one possibility for how life began (Monnard and Deamer 2002), postulating that protolife began in a lipid-protected environment. Membranes of current cells are composed of amphipathic phospholipids. A major challenge for theories suggesting such a lipid bilayer for protolife is that phosphate was essentially unavailable for prebiotic chemistry due to its insolubility in the presence of divalent cations that were likely present (Pasek 2008). Despite the advantage that a lipid membrane provides for establishing a protected chemical environment for protolife, it seems unlikely that phospholipids formed the earliest membranes.
Alternatively, a peptide world is postulated in which peptides have been recognized as central to the origin of life. Since peptides currently provide numerous biological functions, their necessity early in the development of life has been obvious. The coupling of amino acids to form peptides has been extensively investigated. Formation of peptides by dehydration condensation of amino acids has included wetting/drying cycles (Schwendinger 1995), high temperatures (Fox and Middlebrook 1954; Shock 1993; Sakata, Kitadai and Yokoyama 2010), high pressures (Otake 2011), adsorption (Gururani, Pant, Pandey and Pandey 2012; Lambert 2008), hygroscopic salts (Kitadai, Yokoyama and Nakashima 2011), activating agents (Hulshof and Ponnamperuma 1976; Brack 1982), dehydrating agents (Lambert 2008) and near saturation of water with sodium chloride combined with copper(II) salts (Lahav and Chang 1982; Rode and Schwendinger 1990; Rode, Fitz and Jakschitz 2007). Although there is no general agreement on which conditions were relevant in producing early peptides, several conceivable candidate conditions have been described in which peptides' amide bonds could have formed from the ammonium and carboxylate groups of amino acids.
Currently, cellular membranes are composed of about half lipids and half proteins (Rondel et al. 2009) and their co-evolution is likely (Mulkidjanian, Galperin and Koonin 2009; Mulkidjanian and Galperin 2010). Cells of many bacterial (Asselineau 1991) and eukaryotic species contain a ubiquitous amphipathic group of lipoamino acids, lipopeptides and lipoproteins. Their presence in Archaea is dubious (Dibrova, Galperin and Mulkidjanian, 2014; but see Bodour, Drees and Maier 2003; Kebbouche-Gana et al. 2009). These compounds typically consist of a fatty acid bound to the amino acid or peptide by an amide bond. The widespread occurrence in at least two of the three domains is possibly indicative of life's conservation of these biomolecules that have proven to be of significant advantage throughout evolutionary time.
Dehydration condensation reactions of amino acids are likely similar to reactions between fatty acids and amino acids that could also form amide bonds. Similar reaction conditions could have generated peptides along with lipoamino acids and lipopeptides on the early Earth.